According to 2019 American Heart Association Heart Disease and Stroke statistics (Benjamin, et al., 2019) cardiovascular diseases are responsible for most of the deaths in the United States. These diseases are found in 121,000,000 US citizens over the age of 20, or 48% of the adult population. All of these cardiovascular diseases are linked to cellular hypoxia. Cellular hypoxia is defined as a relative deficiency in the availability or utilization of oxygen in a living cell compared to normal physiological conditions.
The annual costs of cardiovascular diseases in the United States will reach $1.1 trillion in 2035. Despite tremendous progress in the management of cardiovascular diseases these same diseases remain the most frequent causes of death in the US, and must be considered a worldwide problem as well. In 2016, about 17.6 million deaths were attributed to cardiovascular diseases globally, an increase of 14.5% from 2006.
Health professionals point with some pride to the substantial decreases in the death rate due to cardiovascular diseases in the US in recent years. However, the patients who once died of these diseases acutely have now become chronic patients in need of long term management. The challenge to us as a society is to devise low cost management strategies that will address this growing epidemic.
When oxygen is in short supply, cellular metabolism and therefore life itself is jeopardized. Living cells respond to a change in the amount of ambient oxygen available with an exquisitely sensitive mechanism that reprograms pathways of gene expression. The nature of the response is particular to the context and environment of each cell. Hypoxia inducible factors (HIFs) are found in organisms ranging from primitive worms to humans, wherever delivery of oxygen is an important variable in the life of an organism, and there are many current pharmaceutical development programs devoted to alteration of HIF physiology. (Semenza, 2019) HIFs provide us with an important illustration of a key component of hypoxia-mediated gene expression, but the range of human cellular responses to oxygen is not limited to modulation of HIF expression.
Hypoxia is a correlate of hypoperfusion, and hypoxia is noted in cardiovascular diseases, as well as other conditions associated with decreased blood flow to a perfused organ. Reperfusion is associated with the return of blood flow. Reperfusion is ordinarily desirable, but prolonged ischemia and subsequent return of blood flow can lead to local inflammation and cell injury due to toxic byproducts of oxidative metabolism. There is a way to mitigate this process. Repetitive brief hypoperfusion and reperfusion can increase the tolerance to future ischemic events. This phenomenon is known as preconditioning. An effective treatment for the adverse consequences of nonlethal severe decreases in oxygen delivery and perfusion has been highly sought after and the therapeutic options are limited at this time.
Most compounds presently under clinical development for management of hypoxia-related gene expression inhibit the prolyl hydroxylases. Prolyl hydroxylases cause hypoxia inducible factor to breakdown. If you inhibit prolyl hyrdoxylases, then you inhibit the agent that causes breakdown of HIF, and the double negative in effect potentiates HIF. Prolyl hydroxylase inhibitors are compounds that potentiate the adaptive response of cells to hypoxia on a systemic basis via the HIFs. HIF, potentiated by prolyl hydroxylase inhibitors, can cause the kidney to synthesize extra erythropoietin, an endogenous molecule that stimulates red blood cell development to therapeutic effect in the setting of anemia. HIF potentiation also leads to angiogenesis, leading to greater blood vessel growth to hypoxic regions, which can be beneficial when, for example, coronary artery disease leads to myocardial ischemia and associated cardiac dysfunction. Unfortunately, the same global approach may simultaneously increase the arterial supply of dormant malignancies, and/or alter cell metabolism to conserve oxygen where oxygen is already available. It is easy to underestimate the complexity of the pathophysiology of hypoxia-related disease states. A very selective approach to regulation of hypoxia-mediated events will be required to develop safe and effective pharmaceuticals. This will involve delivery within a limited organ-specific space or triggering of activity in the proper pathological context.
The expression of cyclooxygenase-2 (COX-2) is transcriptionally regulated by hypoxia in human umbilical vein endothelial cells in culture via the transactivation factors NF-κB p65, HMG I(Y), and Sp1, leading to increased production of PGE2. (Schmedtje et al., 1997; Xu et al., 2000; Ji et al., 1998) These discoveries reflect the fact that HIFs are insufficient to drive all of the human adaptations to hypoxia, and that NF-κB is another important mediator of hypoxia-driven transactivation of genes in the vascular endothelium.
Ischemic cardiovascular diseases are associated with a lack of blood flow and are also associated with hypoxia. Ischemic preconditioning is a portion of the therapeutic effect attributed to the present compounds. Vascular endothelial expression of COX-2 is increased by hypoxia. COX-2 is essential for ischemic preconditioning as a companion to the synthesis of nitric oxide (NO). (Li, Q. et al., 2007) Both are required in order to achieve the protection of the heart that is conferred by the late phase of ischemic preconditioning. (Guo et al., 2012)
The SLC14 (solute carrier 14) family of urea transporter genes regulate urea transport across cell membranes. UT-B (urea transport protein, B, the product of the gene SLC14A1) facilitates transport of urea, water and urea analogues across cell membranes. (Shayakul et al., 2013) UT-B is expressed widely, including in the heart, vascular endothelium and erythrocytes. UT-B null mice have cardiac conduction abnormalities, increased brain urea concentration and decreased NO production. (Li, X. et al., 2012) Urea is freely permeable and enters cells passively, but the equilibrium is slow, and UT-B facilitates the rapid expulsion of urea from erythrocytes. (Sands, 1999) Urea is generally considered a waste product and it carries nitrogen from the breakdown of amino acids to recycling opportunities. Renal failure is associated with decreased nitric oxide synthase (NOS) activity. However, rats with normal renal function do not have decreased NOS activity when BUN is raised to uremic levels. (Xiao et al., 2001) Urea may have cardioprotective properties in some contexts (Wang et al., 1999)
Membrane UT-B is abundant on human vascular endothelium in culture derived from various locations and appears to participate in regulation of nitric oxide (NO) synthesis. (Wagner et al., 2002) Pharmacological inhibition of UT-B in the vascular endothelium causes intracellular accumulation of urea. This is believed to lead to a feedback inhibition on arginase (the enzyme that converts 1-arginine to urea) that elevates the activity and expression of the alternative pathway for 1-arginine, in this case endothelial NOS, (eNOS) enabling increased production of NO. (Sun et al., 2016)
Hypoxia increases the expression of UT-B in hypoxic vascular endothelium. The inventor found that messenger RNA for the gene for UT-B (SLC14A1) is significantly upregulated in human vascular endothelial cells in hypoxic (1% oxygen) cell culture. The upregulation of expression of UT-B in this setting will pump urea out of the endothelial cell and remove the feedback inhibition of arginase that causes increased eNOS activity, reducing the net vasodilator effect of nitrate administration. A source of urea or a urea analogue should maintain effective intracellular urea substrate levels and route 1-arginine to eNOS production of NO, augmenting vasodilatation in response to hypoxia, dilating adjacent vascular smooth muscle.
It would be beneficial to discover compounds that potentiate vasodilatory release of NO in hypoxia while enabling the process of ischemic preconditioning in a targeted (e.g., local) manner, thus leading to the treatment and prevention of major adverse cardiac events in cardiovascular diseases.